1. Field of the Invention
The invention relates to pistons of aluminium or aluminium alloy for internal combustion engines.
Pistons of aluminium or aluminium alloy have been used extensively for many years in internal combustion engines. They have the advantage, in comparison with pistons of ferrous materials, of being light in weight and so offer improved performance. However, aluminium or aluminium alloys are, in general, not as resistant to wear as ferrous materials and this produces a problem in relation to the piston rings carried by such pistons. These piston rings are commonly made of ferrous materials and are received in respective piston ring grooves extending around the piston. During reciprocation of the piston, the piston rings move in their associated grooves and tend to cause wear of the grooves, which can lead to loss of performance of the associated engine and, eventually, to piston failure.
2. Review of the Prior Art
In view of this problem, it has been customary to incorporate into aluminium or aluminium alloy pistons, a piston ring carrier of a material more wear-resistant than the aluminium or aluminium alloy. For example, piston ring carriers of ferrous materials are commonly used. The piston ring carrier is provided with one or more piston ring grooves for receiving associated piston rings and, because of the improved wear-resistance of ferrous materials, as compared with aluminium or aluminium alloys, the wear problem between the ring and groove is at least mitigated.
Because such carriers are formed separately from the piston and are encast during manufacture of the piston, it has been the practise to ensure that there is no possibility of relative movement between the carrier and the piston, particularly in radial directions relative to the axis of the piston and in directions parallel to the axis of the piston. The reason for this is that it has been thought that if such movement were to occur, there would be fretting and wear between the mating surfaces of the carrier and the piston body and that this would lead eventually to such movement of the carrier as would damage either the piston body and/or the associated cylinder or liner. There are numerous proposals for preventing such movement. There is the well known and widely used AL-FIN bond in which the carrier is made of austenitic cast iron (which has a similar coefficient of thermal expansion to aluminium or aluminium alloys) and in which a positive bond is created between the carrier and the piston body. In addition, there are numerous examples of carriers which are provided with formations which form a positive mechanical interlock between the carrier and the piston body. Examples of these are shown in British Patent Specifications Nos. 1 283 956 and 548,400.
Of course, all these proposals increase the manufacturing cost of the piston and, where mechanical interlocks are provided, increase the weight of the piston, so decreasing its performance. In particular, the use of an AL-FIN bond increases the cost of piston production, because austenitic cast iron is more expensive than normal cast irons such as grey cast iron, and because austenitic cast iron is more difficult to cast and machine than other cast irons.